The present invention relates to a novel flexible copper-coated laminate and a flexible printed circuit board. More particularly, the invention relates to a flexible printed circuit board which has distinguished bending characteristic, and a novel flexible copper-coated laminate which can produce the flexible printed circuit board.
Conventionally, taking advantage of own flexibility, flexible printed circuit boards (hereinafter merely referred to as FPCs) have mainly been used in narrow space inside of cameras in folded form. However, recently, in addition to conventional cameras, FPCs have also been used to compose slidable portion for linking drive-reading head with head-controlling unit of electronic apparatuses such as floppy-disc drive (FDD) and hard-disc drive (HDD) for example. Development to achieve further down-sizing and power-saving of electronic apparatuses has been underway, and thus more substantial bending characteristic is required for FPCs. To achieve higher bending characteristic, further thinning of FPCs has been studied by way of thinning component material layers.
As an example of conventional FPCs, a triple-layer FPC comprising the laminate of a polyimide film layer, an adhesive-agent layer, and a copper layer is indicated. Typically, as shown in FIG. 3, such a triple-layer FPC is manufactured by a process of manufacturing triple-layer flexible copper-coated laminate 4 by adhering polyimide film 1 to copper foil 2 via adhesive-agent layer 3, followed by a process of forming circuits by etching copper layer (2) secured to the copper-coated laminate 4.
However, since the adhesive-agent layer 3 is present in the triple-layer FPC (4), there is a limit in the thinning. In addition, since the adhesive-agent layer 3 uses adhesive agent 3 exerting less heat-resistant property, electrical characteristic, and mechanical strength than those of the polyimide film 1, thus there has been such a problem as proper characteristic of the polyimide film cannot fully be exerted. To solve the above defective property of triple-layer FPCs, such a double-layer FPC comprising polyimide a film layer and a copper layer for example without presence of an adhesive-agent layer has drawn attention of the concerned. Actually, development of the double-layer FPC has already been underway. Since no adhesive-agent layer is present, the double-layer FPC can substantially exert proper characteristic of polyimide film, and yet, laminate can be thinned further than the above-cited triple-layer FPC to permit the double-layer FPC to improve heat-resistant property and bending characteristic.
The double-layer FPC can be manufactured by initially forming a flexible copper-coated laminate comprising double layers consisting of a polyimide film layer and a copper layer, followed by a process to form a circuit by way of etching the copper layer thereof. The double layer flexible copper-coated laminate can be manufactured by applying any of those methods including the following; cast method which initially coats a single surface or both surfaces of copper foil with varnish of polyamide acid being a precursor of polyimide, followed by a process to imidize the varnished surface by applying heat treatment; chemical vapor deposition method which thermally vaporizes copper component in high vacuum condition followed by a process to cause vaporized copper component to be adhered onto polyimide film surface as thin film; or non-electrolytic plating method which initially causes copper to be precipitated on a polyimide film surface by generating chemical reduction reaction in plating solution before eventually forming a copper layer thereon.
Nevertheless, because of handling inconvenience of material such as polyimide film or copper foil, there has been the following problem in the course of manufacturing the double-layer copper-coated laminate, and as a result, it was impracticable to diminish thickness of respective materials to 10 .mu.m or less than 10 .mu.m.
Concretely, when manufacturing a double-layer flexible copper-coated laminate via the above-cited cast method, because of the need to coat a single surface of copper foil with varnish of polyamide acid, handling property of copper foil must securely be maintained, and thus the thickness of copper foil could not be reduced to 10 .mu.m or less than 10 .mu.m. On the other hand, when manufacturing a double-layer flexible copper-coated laminate via the above-cited chemical vapor deposition method or non-electrolytic plating method, since the surface of polyimide film is treated with chemical vapor deposition process or plating process, handling property of polyimide film must also be maintained, and as a result, there has been such a problem that the thickness of polyimide film cannot be diminished to 12.5 .mu.m or less than 12.5 .mu.m.
Because of the above reasons, it was impracticable to manufacture such a double-layer FPC having a thickness of 10 .mu.m or less than 10 .mu.m of polyimide film and copper layer, thus resulting in the difficulty to materialize thinner layers of FPCs.
In order to fully solve the above problems by way of realizing thinner layers of double-layer FPCs and provide a flexible copper-coated laminate capable of manufacturing such FPCs distinguished in bending characteristic and heat-resistant property, inventors have strenuously followed up studies and discovered that such a film composed of polyimide polymer having modulus of initial tensile elasticity of 400 kg/mm.sup.2 or more than 400 kg/mm.sup.2 can exert sufficient handling property even when the thickness of the film is 10 .mu.m or less than 10 .mu.m, and eventually consummated the invention.